A comparison of the thermal gradient and elution methods of polymer fractionation

Abstract

The elution method is essentially a single-stage process, whereas the thermal gradient method is a multistage process which depends upon a thermal gradient to bring about reprecipitation of polymer in the fractions. As a test of the effectiveness of the thermal gradient, comparisons have been made of fractionation by these two column methods on high molecular weight polystyrene samples. It was found that the thermal gradient method definitely provides superior resolution and reproducibility, as expected. However, the degree of fractionation obtained by the elution method was surprising, accounting for at least 80% of the sample under the usual conditions and giving complete fractionation with certain modifications of conditions. These results indicate the difference in performance of the two methods is less than expected from an elementary consideration of the operation of the columns, and fractionation by the elution method, as conducted here, exceeds that expected for a single-stage extraction process. Although the reasons for the observed behavior are not clear, the following conclusions have been reached about certain factors which influence fractionation. Alternative methods of controlling the concentration of polymer in the fractions give almost equivalent results but enhanced resolution of the high molecular weight portion of the sample is obtained with extended solvent gradients. The inhibitor, tert-butyl catechol, which it was necessary to add to the solvents to limit degradation of the very high molecular weight sample, plays a specific role in the fractionation due to a reaction with the polystyrene which alters the fractionation behavior without affecting the molecular weight. Also, trace amounts of chemical heterogeneity in the polymers, presumably hydroxyl groups, have a marked adverse effect on fractionation by the elution method and probably account for molecular weight reversals observed in some fractionations by the thermal gradient method. It is suggested that adsorption on the surface of the beads is responsiblp for the adverse effect of chemical heterogeneity on the fractionation and that possibly an adsorption which increases with molecular weight contributes to fractionation by the elution and thermal gradient methods.